Automatically shiftable transmissions used in transportation vehicles, such as cars, buses, and trucks, require a positive displacement pump to supply pressurized hydraulic fluid for torque transmitting mechanism (clutches and brakes) engagement, torque converter operation, and cooling flow.
The pump requires power from the engine or prime mover to supply the required control pressure. The power absorbed by the pump and therefore supplied by the engine is a function of the pressure and displacement of the pump. The higher the pump output pressure or main pressure of the transmission, the more horsepower required from the engine.
Current transmissions utilize control mechanisms having electronic systems. The electronic systems are supplied with signals from the engine, vehicle, and transmission. The signals are utilized to determine the operating parameters of various solenoid valves within the control system to modulate various pressures including the main pressure or line pressure of the transmission. By modulating the main pressure, the efficiency of the vehicle may be improved.
Additionally, when the vehicle comes to a stop, while in gear, the torque converter slips to decouple the engine from the remainder of the drivetrain. This slipping within the torque converter may decrease operating efficiency of the vehicle as well as cause the hydraulic fluid contained within the torque converter to heat.